WARNING: This story contains a graphic image of an unidentified man believed to be dead

In a rare move on Monday, Toronto police released a photograph of an unidentified man who investigators believe is another victim of alleged serial killer Bruce McArthur.

"I did not want to release this picture and I am doing so as a last resort," said Det.-Sgt. Hank Idsinga, lead investigator in the case.

"We have utilized numerous investigative techniques to identify this individual and so far have been unsuccessful."

Idsinga declined to comment on the origin of the image or how it came into the possession of investigators. He said that it had been altered slightly to make the photograph sharper and to remove "artifacts" from the background.

"We need to put a name to this face and bring closure to this man's loved ones," he told reporters at a morning news conference at Toronto police headquarters.

Idsinga also confirmed that a seventh set of remains were found by investigators at a midtown Toronto home where McArthur worked as a landscaper. The remains have not been identified, and Idsinga made it clear they have not been linked to the unidentified man in the photograph.

Toronto police say they released an image of an unidentified man believed to be a victim of Bruce McArthur as a 'last resort.' 0:18

McArthur, 66, now faces six counts of first-degree murder. He was arrested on Jan. 18 and charged in the cases of Andrew Kinsman, 50, and Selim Esen, 44. The other charges were laid later that month, involving Majeed Kayhan, 58, Dean Lisowick, 47, Soroush Mahmudi, 50 and Skandaraj Navaratnam, 40.

He is currently being held at the Toronto South Detention Centre in suburban Etobicoke.

The Mallory Crescent property where the self-employed landscaper worked and stored his tools has been ground zero for the investigation, with at least six other sets of remains found in planters there.

Three of the sets have been identified as belonging to Mahmudi, Kinsman and Navaratnam.

This combination of photos shows the six men McArthur is charged with killing. Top row, from left to right: Skandaraj Navaratnam, 40, Andrew Kinsman, 49, Selim Esen, 44. Bottom row, from left to right: Dean Lisowick, Soroush Mahmudi, 50, and Majeed Kayhan, 58. (CBC/Toronto Police Service)

Forensic pathologists have struggled to identify the remaining four sets of remains and determine the cause of death in each instance, according to Idsinga. While police completed their search of "up to 20" planters that McArthur was known to use weeks ago, it only recently became evident that remains of a seventh person were among those previously discovered.

"I can't really get into specifics about why we didn't realize there were seven remains there, but we had the planters several weeks ago," he told reporters

Finger print and dental analysis both failed to yield any results, and it could be months before the more rigorous DNA analysis process is completed. ​

While Idsinga has an idea of how at least some of McArthur's alleged victims were killed, he said investigators have been unable to definitively determine the cause of death in each case.

Michael Pollanen, Ontario's chief forensic pathologist, called the McArthur case a "unique investigation in the history of our organization," and pointed to a number of challenges that have impeded progress.

Michael Pollanen, Ontario's chief forensic pathologist, on Monday called the McArthur case an 'investigation that is unique in the history of our organization.' (David Donnelly/CBC)

"First of all, decomposition can affect how accurately we can determine the cause of deaths," he told reporters at the news conference.

McArthur's earliest known victim, for example, was first reported missing in 2010.

Secondly, Pollanen added, "if you have an incomplete set of remains, it may be very difficult to ascertain the cause of death."

Idsinga said previously that investigators are probing missing persons cases dating back decades. There are also a number of other properties in the Greater Toronto Area that will get renewed attention from police as the weather warms and the ground thaws.

Toronto police Det.-Sgt. Hank Idsinga released a photograph of an unidentified man that he believes is another of Bruce McArthur's victims. (David Donnelly/CBC)